The present application claims priority to German Application No. DE 102 12 677.1, filed on Mar. 22, 2002, and German Application No. 102 12 679.8, filed on Mar. 22, 2002, both of which are hereby incorporated by reference in their entirety.
1. Field of the Invention
The present invention provides an aqueous dispersion containing at least two powder types selected from one or more metal oxide powders and/or one or more non-metal oxide powder. The present invention also provides a coating composition containing this dispersion, an inkjet recording medium containing the coating composition, and methods of making the same.
2. Discussion of the Background
In order to improve the print properties of the surfaces of absorptive supports, recording media may be coated with a coating composition. The particularly important print properties to be improved include: adsorption, drying time, ink adhesion, and recording medium glossiness. For photograph-type materials glossiness and high ink absorption capacity are of particular importance.
In general, the coating composition utilized to produce a glossy absorptive support contains an aqueous dispersion of pigments (e.g., hydrated aluminium hydroxide, aluminium oxide, silicon dioxide (silica), and/or titanium dioxide) and a binder (e.g., polyvinyl alcohol), whereby the pigments are incorporated in the form of powders or as a dispersion of powders.
High-gloss coatings are often obtained by employing fine silica particles. However, the dispersions used for these coating compositions often have low stabilities and high viscosities. Accordingly, the dispersion often has to be produced immediately before being used in a coating composition. Due to increased viscosities, more highly filled dispersions are difficult to process.
The filler content used in the coating composition is an important parameter for controlling the quality of the recording medium and for the economic efficiency of the ink recording processes. As the filler content in the coating composition increases, the amount of the coating composition needed to obtain a specific rate of application decreases. In addition, less water has to be evaporated with a high filler content, which leads to faster drying. Accordingly, the process can be performed more economically than with a coating composition having a low filler content.
A high gloss and a good ink absorption capacity can also be achieved through processing if the coating composition is applied by cast coating. However, this process is relatively slow and cost-intensive.
In DE-A-100 35 054 cationised fine silica particles with a primary particle diameter of 50 nm or less are used in an aqueous dispersion to produce a coating composition that gives rise to a recording medium with high gloss and good ink absorption capacity.
U.S. Pat. No. 6,284,819 describes a coating composition with a specific viscosity that is obtained from an aqueous dispersion of two particles differing in type and size. The first powder type comprises metal oxide particles such as silica, cationised silica or aluminium oxide. The particle size is defined in that the first powder type comprises aggregates of smaller primary particles having an average primary particle size of less than 100 nm and an average aggregate size of 100 to 500 nm. The second powder type comprises metal oxides and synthetic polymers. The average aggregate diameter of the particles in the second powder type is at least half the size of the average aggregate diameter of the first powder type. The ratio by weight of the particles of the first to the second powder type is between 9 and 91 wt. %.
By employing this coating composition, a recording medium with high gloss and good ink absorption capacity can be produced. The first powder type of particles should be responsible for the absorption of liquid. The smaller aggregates of the second powder type should fill voids. Overall the packing density of the coating is increased. The substantial feature is that the average aggregate diameter of the particles of the second powder type is at least half the size of the average aggregate diameter of the first powder type.
As is shown in the embodiment examples, the coating composition is obtained by adding a binder, such as polyvinyl alcohol, to a physical mixture of two aqueous dispersions, one dispersion containing the particles of the first powder type, one dispersion containing the particles of the second powder type. All combinations of metal oxide particles, regardless of their specific surface charge, at a specific pH of the dispersion are disclosed in U.S. Pat. No. 6,284,819. However, this combination can lead to dispersions that are not stable and that rapidly gel. Therefore, the coating compositions of U.S. Pat. No. 6,284,819 are only of limited suitability for production of a coating composition.
Accordingly, there remains a critical need for coating compositions and absorptive media produced using these coating compositions that have high glossiness, good ink absorption capacity and rapid drying times. Of particular importance are dispersions that serve as the starting material for these coating compositions.
Accordingly, it is an object of the present invention to provide a dispersion with a high filler capacity and low viscosity. It is also an object of the present invention to provide a coating composition containing these dispersions that, when applied to an absorptive support, produces a recording medium having a high gloss, good ink absorption capacity and good drying performance.
The present inventors have found that the object of the present invention is achieved by using a stable, aqueous dispersion containing 20 to 80 wt. %, relative to the total amount of dispersion, of one or more metal oxide and/or non-metal oxide powders,
which are present in the dispersion in the form of aggregates of primary particles,
whereby the average diameter of the aggregates is 10 nm to 300 nm and
the average diameter of the primary particles is 5 to 50 nm,
wherein
the dispersion contains at least two types of powder,
whereby the powder types at a given pH of the dispersion display the same surface charge sign;
have a zeta potential that gives rise to an electrostatic repulsion between the particles that is greater than the van der Waals attraction between the powders;
the average diameter of the primary particles differs by a factor of at least 1.5 between two powder types;
in the dispersion the average aggregate diameter of the second and additional powder types displays 60 to 150% of the size of the first powder type; and
each powder type is present in the dispersion in a quantity of at least 1 wt. %.
In an embodiment of this object of the invention, two types of powder are present. Further, any powder(s) present may be produced by flame hydrolysis.
In another embodiment of this object of the invention, the weight ratio of the first powder type to the second powder type in the dispersion is 10:90 to 90:10.
In another embodiment of this object of the invention, the average primary particle diameters of the powder types in the dispersion differ by a factor of at least 2.
In still another embodiment of this object of the invention, the average aggregate diameter of the second and additional powder types in the dispersion is 80 to 120% of the size of the first powder type.
In an embodiment of this object of the invention, the viscosity of the dispersion does not exceed a value of 1500 mPas at a shear rate of 12 sxe2x88x921 and a temperature of 23xc2x0 C.
In yet another embodiment of this object of the invention, the dispersion may also contain one or more substances to adjust the pH (such as an acid, a base, and a buffer system) and/or one or more additives selected from the group consisting of a salt, a surface-active substance, an organic solvent, a bactericide and a fungicide.
In other objects of the present invention are process for the production of the inventive dispersion, coating compositions containing the inventive dispersion, methods of making the coating composition, a recording medium containing the coating composition, and methods of making the recording medium.
The above objects highlight certain aspects of the present invention. Additional objects, aspects and embodiments of the present invention are found in the following detailed description of the present invention.